The present invention relates, in general, to low dust adsorbents and catalysts and the method for their manufacture.
A disadvantage to the use of industrial adsorbents and catalysts such as activated alumina and molecular sieves is their dustiness. Adsorbent and catalyst manufacturers and users need to reduce the level of dust and fine particles that are created in their manufacture and use in order to meet environmental and safety standards and also to meet process requirements. Dust and fine particles can interfere with the process of manufacture and use by plugging filters and valves, creating excessive pressure drops across the system and cause quality problems with products.
There are several factors that can cause a material to be dusty. Compaction of powders in the presence of liquids and additives by agglomeration and extrusion is the most popular production method for production of porous adsorbents and catalysts. Inevitably, some powder adheres to the outside surface of the final particle and can easily escape from it in the form of dust. Static charges can also cause dust accumulation on the particle surface. In addition, the unloading and packaging operation can generate dust by abrading any loosely bound particles at the granule periphery of a granule.
There are several different methods that can be applied to reduce the dustiness of solid materials that are most commonly present in the form of beads, granules or extrudates. Removal of dust and prevention of dust formation are the principal measures taken. Extensive screening in re-roll equipment, fluidizing and blowing with a gas flow are all common methods to cope with the dust problem. Washing followed by activation or further handling is another possible approach. For example, U.S. Pat. No. 5,637,349, which is hereby incorporated by reference, describes a system and a process for reducing the dust formation during the transfer of a particulate material on an endless conveyor belt. A wetting agent selected from the group consisting of water and organic surface active compounds is used to reduce the dusting.
Several different approaches can be applied to improve the attrition/abrasion resistance of the material and to diminish the intrinsic ability for dust formation. Among the disadvantages of the prior art methods are the danger of damaging the outside surface of the particulates by extensive rolling and screening, which can lead to damaging of the pores employed in adsorption and catalysis. There may be a need for additional thermal treatment or a change in the properties when washing is used as a dust removing technique. A material that has been treated to remove dust may still generate new dust upon subsequent packaging, transportation and unloading at the final destination.
There is a need for low cost, effective adsorbents for use in pressure swing adsorption air dryers, as well as in other applications. A suitable adsorbent must not only produce low dew points, but it must also have acceptable physical properties, such as low dust content and resistance to abrasion loss and attrition. Air brake drying represents a severe environment for the operation of a dryer due to the desiccant being subjected to vibration and due to the large pressure swings between the adsorption and desorption steps. In addition, the desiccant is exposed to hot compressed air, which may contain oil, and other volatile compounds such as hydrocarbons than can harm brake cylinders and related equipment. Adsorbents are sought which are more resistant to attrition and abrasion loss than conventional materials and which are effective in the removal of water and other contaminants from the compressed gas to produce dry compressed gases.
In a related U.S. patent application, Ser. No. 09/796,218 filed Feb. 28, 2001, it was disclosed that dust could be reduced by spraying cured, alumina balls, prior to activation with a silica coating. Surprisingly, it has now been found that the dust evolution from an adsorbent or catalyst can be greatly diminished by treatment with a small amount of colloidal silica on activated balls of alumina or on calcined balls of molecular sieve. In addition, it has been found that the process of the present invention can be applied to a wide variety of different materials without loss in performance, heat release or material damage. Furthermore, it was found that there was no need for heat treatment aside from brief drying after application of the coatings of the present invention. The improvement in lowered dustiness is combined with an enhanced resistance for dust formation in subsequent operations.
According to the present invention, activated shaped or pre-formed adsorbent or catalyst particles are modified by depositing a thin dispersed layer of a colloidal solution of a metal oxide such as silica, alumina, zirconia or combinations thereof on the external surface of the particles. The colloidal silica coating drastically reduces the abrasion loss and dusting. Surprisingly, this improvement is achieved without seriously reducing the performance of the adsorbent or catalyst particles.
Accordingly, in one embodiment, the present invention relates to a particle selected from the group consisting of adsorbents and catalysts wherein the particle comprises an adsorbent or a catalyst and wherein the particle is selected from the group consisting of porous refractory oxides such as alumina, silica, silica-alumina, zirconia, silica-zirconia, titania, magnesia and combinations thereof and zeolite and other types of molecular sieves. Dispersed on the particle is a metal oxide layer derived from colloidal solution in which at least 60 percent of the metal oxide is present in a band extending from the surface towards the center and having a width of less than 50 microns.
In another embodiment, the present invention relates to a method for producing a catalyst or an adsorbent comprising forming and activating a catalyst or an adsorbent particle; and coating the catalyst or adsorbent particle with a coating solution comprising colloidal metal oxide to provide a narrow layer of metal oxide dispersed thereon.
Additional objects, embodiments and details of this invention can be obtained from the following detailed description of the invention.